This invention relates generally to radiation emitting sources for sensing targets and, more particularly, to radiation emitting sources, for example radar systems, which emit radiation and sensing reflections for target detection.
In the past, there have been known methods employed for locating radar emitter sources, which required the knowledge of several parameters related to the operation of the emitter source measurements made from two or more known spatial locations, and/or the known rate of relative motion of the emitter source. Conventional radar sources can be vulnerable to countermeasures because the emitted radiation possesses rather simple phase fronts. The countermeasures that can be employed by an aircraft that is being imaged include sending a missile down a course perpendicular to the phase fronts to destroy the radar installation, or sending back “rogue” radiation with an altered phase front pattern to suggest a reflection from elsewhere. Each of these methods require an assessment of the phase front emitted by the source, which is easier to perform if the phase front lacks complexity.
The principles of phase-front analysis may be understood by considering a very simple phased array consisting of three dipole antennae at positions whose cylindrical polar coordinates (r, φ, z) have the values (0, 0, 0), (a,+π/2, 0) and (a,−π/2, 0)
If the dipole antennae are excited in phase at an angular frequency ω, it is simple to show that the signal received at time tP at a far-field observation point P with the spherical coordinates (RP>>a, θP=π/2, φP) will have the amplitude
                              E          ∝                                    1                              R                P                                      ⁢                                          cos                ⁡                                  [                                      ω                    ⁡                                          (                                                                        t                          P                                                -                                                                              R                            P                                                    c                                                                    )                                                        ]                                            ⁡                              [                                  1                  +                                      2                    ⁢                                                                                  ⁢                                          cos                      ⁡                                              (                                                                              ω                            ⁢                                                                                                                  ⁢                            a                            ⁢                                                                                                                  ⁢                            sin                            ⁢                                                                                                                  ⁢                                                          φ                              P                                                                                c                                                )                                                                                            ]                                                    ,                            (        1        )            
where c is the speed of light in vacuo, and the origin of the time coordinate is chosen to remove any arbitrary phase. Note that the time-independent part of this expression has zero crossings at
                                          φ            P                    =                      arcsin            ⁡                          [                                                                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    c                                                        ω                    ⁢                                                                                  ⁢                    a                                                  ⁢                                  (                                      j                    ±                                          1                      3                                                        )                                            ]                                      ,                            (        2        )            where j is an integer (see FIG. 1 for a plot of E2).
Now consider two closely-spaced detectors, labelled P and Q and placed at positions (RP, π/2, φP+δφP) and (RQ, π/2, φP−δφP). The signals VP and VQ detected by P and Q will be of the following formVP=AP cos(ωtP−γP) and VQ=AQ cos(ωtP−γQ),  (3)where γP and γQ are phases that depend on the relative positions of P and Q. If the detectors are identical in all respects, and RP=RQ, then γP−γQ vanishes for all φP, apart from a very small region of angular width 2δφP around each of the zero-crossings defined by Eq. 2, where |γP−γQ|=π (see FIG. 1). Away from the angles at which zero crossings occur, setting RP≠RQ introduces a phase difference γP−γQ=(ω/c)(RP−RQ).
The surface defined by RP=RQ therefore represents a surface of constant phase, or “phase front”.
The detection of such phase fronts, by appropriate positioning of P and Q, constitutes a method for locating the direction of the phased array. Although modifications can be made to the relative phases of the elements of a phased array to “steer” the beam or apply an apparent slant to the phase front, the very regular nature of the phase fronts that emanate from such systems makes them vulnerable to being located and subjected to countermeasures.
An essential part of warfare involving radiation sensing systems is the location of targets or reference marks by means of a detection system such as radar, and it is common that any potential enemy will take steps, such as the creation of interference utilizing countermeasure devices to prevent the effective use of detection equipment against targets. Such countermeasure devices can assume various forms such as for example an inverse gain repeater, a range gate pull-off repeater, chaff, radar decoys, image frequency jammers, and other forms. A usual target that utilizes such countermeasure technology is an aircraft.
In a known method, knowledge of the waveform modulation of the emitter source in the time or frequency domain can be utilized. An example of such a requirement included the scan rate, the pulse duration, the pulse interval and/or the frequency modulation patterns. In another known method, measurements can be provided in the form of the emitter signal angle of arrival or the emitter signal time of arrival. Measurements in the form of angle of arrival can be utilized in the process of triangulation. The emitter signal measurements of angle and time of arrival can then be employed in conjunction with the time and location of the measurements to ascertain emitter location. Yet another known method utilized for determining the range to a radar emitter involved the measurement of angular rates between the emitter source and the measurements site/platform.
In the field of microwave radar, a technique has long been used in which an interrogating radar signal is deceived by returning a distorted signal having a discontinuity or other alteration in the phase front, so that it appears to be coming from a different point in space. The art has long sought an equivalent for laser radar. The detection of the location of a radar source by way of analysis of a phase front and the ability to alter the phase front so that it appears that the target is at a different point, is made possible due to the non-complex nature of a conventional phase front. A more complex phase front is needed to avoid some of these countermeasure techniques.